Nadeau and Dolenc (2005) found that
changes in tremor and micro-earthquake rates at Cholame appeared
to correlate. This suggests that deep deformation
associated with the Cholame tremors (i.e., ETS) may also be stressing
the shallower seismogenic zone in this area. Subsequent monitoring
of the Cholame tremors has revealed
further evidence for such stress-coupling. Of particular note have been the
rate changes associated with the 22 December 2003,
M6.5 San Simeon, CA and the 28 September 2004, M6 Parkfield, CA
earthquakes (epicenters 50 km west and 10 km
NW of Cholame, respectively) that have now been observed (Fig.
2.14).

Between 1 and 3 months before the Parkfield earthquake,
tremor activity was relatively low, near pre-San Simeon levels.
The activity then spiked between 20 and 22 days prior to the
Parkfield mainshock. The relationship of this fore-tremor (FT)
to the Parkfield mainshock is suggestive
of coupling between deep stress changes associated with the tremors
and stress changes in the shallower seismogenic zone leading to the
Parkfield M6.0 mainshock.

More profound, however, has been the large and long lasting increase in
overall tremor rates following the Parkfield event. Immediately
following the the mainshock, tremor rates increased to unprecedented
levels that persisted for several days. For several weeks following
this period tremor rates remained extremely high but decayed rapidly
(similar to the decay of aftershocks in the region).
Then, 80 days after the mainshock, tremor rates appear to
have entered into a new state where overall rates decay
much more slowly and where the dominant pattern of activity exhibits
a pattern of multi-scale quasi-periodic variation (i.e., with periodicities
of 75 and 330 days). This pattern has
persisted up to the time of this report, and it is not yet clear whether
the rate behavior reflects solely the response of the tremor
source region to stress from the Parkfield mainshock
or if mainshock stresses have activated other tremor
related processes (e.g., fluid migration or transient deformation).

In any case, the pattern of tremor rate behavior relative to the San
Simeon and Parkfield events supports the argument that nearby moderate
magnitude earthquakes can stimulate deep NVT activity and that such
events may have a significant impact on the long-term evolution of
NVT activity. In addition, because the Cholame segment of the SAF has
an estimated earthquake recurrence time of 140 years (+93, -69) (WGCEP, 1995), and it is now over 140 years since the Fort Tejon
event, future increases in SAF tremor activity may signal
periods of more rapid stress change and an increased probability for
the next large earthquake on the Cholame segment.

Figure 2.14:
Activity rate history of nonvolcanic tremors detected by the
borehole High Resolution Seismic Network (HRSN) at Parkfield (PF), CA.
Histories span 2240 days (3 years 65 days prior to the PF
mainshock to 2 years 349 days after the event). The number
of minutes of tremor activity for each day is computed, yielding
a time series of activity rates with sampling interval of 1 day.
The time series is then smoothed with boxcars of 4 different window
lengths (panel A, 7.5 days; B, 15 days; C, 30 days and D, 60 days)
stepped at daily intervals with values plotted at the center of each
window. This is done to help illustrate the multi-scale
periodicity of the data. Black lines spanning all four panels are times of
the 2003 and 2004 San Simeon and PF earthquakes. Black lines
in panel 1 are times of the 2002 Denali M8.1 and 2004 Sumatra M9.3
earthquakes. Triggering of tremor activity from these global events
appears to be relatively insignificant compared to triggering related
to the San Simeon and PF events. FT refers to apparent fore-tremor
event preceding the PF mainshock. Small vertical dashes (Panel C)
are 75 day intervals approximating periodicity on this scale following
the PF mainshock. Short horizontal lines (panel D) show the approximate
330 day rate pulses.